1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus and, more particularly, to the one in which sheets are separated and fed by blowing an air to the sheets.
2. Description of the Related Art
Conventionally, an image forming apparatuses such as a printer and a copying machine is provided with a sheet feeding apparatus of feeding sheets one by one from a sheet containing portion in which a plurality of sheets is contained. Furthermore, there has been such a sheet feeding apparatus of air sheet feeding type, as described in Japanese Patent Application Laid-Open No. H07-196187, in which air is blown to the end of a sheet stack contained in a sheet containing portion to blow up several sheets, and the uppermost sheet is sucked to a sucking and conveying belt disposed thereabove to be conveyed.
FIG. 11 illustrates one example of a sheet feeding apparatus of such an air sheet feeding type. As illustrated in FIG. 11, a tray 12 on which a plurality of sheets S are stacked is disposed so as to be capable of being lifted or lowered in a storage 11, being a sheet containing portion in which the plurality of sheets S is contained. Moreover, there is provided above this storage 11 a sucking and conveying portion 50A sucking and conveying the sheet S. Furthermore, there is provided on the side of the storage 11 an air blowing portion 30 for blowing air to the end of a sheet stack on the tray 12 to blow up several sheets S, as well as to separate them from one another.
The sucking and conveying portion 50A includes a sucking and conveying belt 21 passed over belt driving rollers 41, as well as sucking and conveying the sheet S rightward in FIG. 11, and a suction fan 36 generating a negative pressure for causing the sucking and conveying belt 21 to suck the sheet S. Further, a suction duct 51 is disposed inside the sucking and conveying belt 21, and acting to suck in air via suction holes formed in the suction belt 21. Furthermore, in order to make ON/OFF of a sucking operation performed by the suction fan 36, a suction shutter 37 is disposed between the suction fan 36 and the sucking and conveying belt 21.
Moreover, the air blowing portion 30 includes a loosening nozzle 33 and separation nozzle 34 for blowing air to the upper portion of the contained sheet stack, a separation fan 31, and a separation duct 32 supplying air from the separation fan 31 to each nozzle 33, 34.
An air having sucked in the direction indicated by arrows C by the separation fan 31 is blown in the direction indicated by arrows D by the loosening nozzle 33, and blown in the direction indicated by arrows E by the separation nozzle 34. Then, by the air blown, several sheets at the upper portion of the sheet stack supported on the tray 12 are blown up, and an uppermost sheet Sa is separated from the blown up sheets.
Now, an initial operation in the sheet feeding apparatus of such construction will be described with reference to FIGS. 12A, 12B, 13A, and 13B. Note that, the initial operation is an operation in which after the refill of sheets and the like, the tray 12 is preliminarily lifted or lowered so that the uppermost sheet Sa of sheets S on the tray 12 is moved to the suckable range where the sheet can be sucked with a sucking and conveying belt 21.
When the initial operation is started, as illustrated in FIG. 12A, the tray 12 on which the sheets S are stacked is lifted. Then, when the distance between the uppermost sheet Sa and the sucking and conveying belt 21 becomes a predetermined distance B, a sheet detecting mechanism (not shown) makes detection to stop lifting of the tray 12.
Subsequently, prepared for a sheet feed signal generated thereafter, as illustrated in FIG. 12B, to loosen sheets, an air is blown to the end of sheets S from the loosening nozzle 33 and the separation nozzle 34. Here, although when an air is blown to the sheets S, upper sheets of the sheet stack are blown up, as illustrated in the encircled portion designated by SG of FIG. 13A, they may be blown up densely. To dissolve the blown up dense sheets, as illustrated in FIG. 13A, an operation of lowering (moving in the direction indicated by an arrow F) the tray 12 is performed after air is blown.
Then, when the tray 12 is lowered like this, as shown in FIG. 13B, the blown up dense sheets are loosened, and the blown up sheets are in the state of being at approximately equally spaced intervals S1. At this time, supposing that the space PB1 between the uppermost sheet Sa and the sucking and conveying belt 21 is an adequate space in which only one sheet can be sucked, the tray 12 is stopped to wait for a sheet feed signal. Upon receiving the sheet feed signal, the sucking and conveying belt 21 sucks the sheet Sa and is rotated, thereby separating and conveying the sheet Sa.
However, in the case of a small basis weight of sheet (thin and light sheet), even if blown up sheets are in the state of being at approximately equally spaced intervals, the space PB1 between the uppermost sheet Sa and the sucking and conveying belt 21 comes not to be larger, and the uppermost sheet Sa may not be positioned with the adequate space relative to the sucking and conveying belt 21. Therefore, there is a possibility that by a suction force of the sucking and conveying belt 21, through the uppermost sheet Sa, the next sheet is sucked, resulting in the occurrence of separation failure. Accordingly, to obtain the adequate space in which only the uppermost sheet Sa can be sucked, the tray 12 continues to be lowered.
Whereby, as illustrated in FIG. 14A, the blown up sheets will be blown up with the space between the uppermost sheet Sa and the sucking and conveying belt 21 becoming larger by degrees, as well as, gradually with the space S2 larger than the space S1 as illustrated in FIG. 13B. Even in the case, however, where the space between the blown up sheets becomes larger, the space between the uppermost sheet Sa and the sucking and conveying belt 21 may not be changed largely. In this case, the tray 12 further continues to be lowered continuously.
Then, by the tray 12 being lowered, the space PB1 between the uppermost sheet Sa and the sucking and conveying belt 21 becomes larger by degrees. As compared therewith, however, when the amount of the tray 12 being lowered comes to be extremely large, the buoyancy of sheets is sharply decreased, and thus the sheets cannot continue to be blown up.
As a result, as illustrated in FIG. 14B, the blown up sheets rapidly drop by a distance N1 in the direction indicated by an arrow F of FIG. 14B. Whereby, the space between the uppermost sheet Sa and the sucking and conveying belt 21 becomes PB2, and thus the space is suddenly changed to be too large. That is, the position of the uppermost sheet Sa will be below the suckable range in which the sucking and conveying belt 21 can suck a sheet.
In this state, the tray 12 stops to be lowered, and will start to be lifted in the direction indicated by an arrow A of FIG. 14B. However, the tray 12 is lifted like this, and thereafter in the case where the sheets S are blown up densely as illustrated in the already-described FIG. 12B, the tray 12 will start to be lowered again. Then, such a lifting and lowering operation is repeated many times.
When a lifting and lowering operation of the tray 12 is repeated many times like this, the uppermost sheet Sa is less likely to be positioned with stability, and will be blown up in an instable state. In case where a sheet feeding operation is started in such a state, which will result in a double feed or jam of sheets.